A conventional quadrupole mass spectrometer is illustrated in FIG. 3A, in which molecules or atoms of a sample are ionized in the ion source 1 and the ions are introduced into the quadrupole filter 3. Ions of a certain mass number (=[mass]/[electrical charge]) selectively can pass through the quadrupole filter 3, and enter the ion detector 4. Since the mass number of ions passing through the quadrupole filter 3 depends on the voltage applied to the quadrupole filter 3, measurement of a predetermined mass range can be performed by changing (scanning) the applied voltage in a specific range. This is the scan-measurement of a quadrupole mass spectrometer.
For example, in a gas-chromatograph mass spectrometer (GC/MS) or liquid-chromatograph mass spectrometer (LC/MS), components of the sample are separated in a course of time by the gas-chromatograph or liquid-chromatograph, while the scan-measurement is continuously repeated at high speed, so that every separated component is scan-measured and the mass spectrographs of the components are obtained. Examples of the voltage scanning patterns are shown in FIG. 3B. In a scan-measurement, it is obvious that the larger the scanning speed is, the shorter the measurement time of a scanning cycle is, and the larger the number of scanning cycles within a predetermined time period is. This means that, in a GC/MS or LC/MS, higher time resolution can be obtained by increasing the scanning speed.
However, a large scanning speed has the following drawback. Supposing that an ion takes the time t1 to pass through the length L of the quadrupole filter 3 as shown in FIGS. 3A and 3B, the time t1 depends on the kinetic energy of the ion when the ion enters the quadrupole filter 3. Because the voltage applied to the quadrupole filter 3 is being scanned, the voltage is changing while the ion is passing through the quadrupole filter 3. This means that the change in the applied voltage while the ion is in the quadrupole filter 3 is larger as the voltage scanning speed is larger.
If the scanning cycle time is very long compared to the time (passing time) needed for the ion to pass through the quadrupole filter 3, the change in the voltage xcex94V while the ion is passing is negligible, and there is no substantial problem. If, however, the voltage change xcex94V is not negligible, some ions that could have otherwise passed through the quadrupole filter 3 cannot pass, and the number of ions reaching the ion detector 4 is less than it should be. This means that the sensitivity of measurement deteriorates as the scanning speed is increased.
The present invention is achieved in this respect. An object of the present invention is therefore to provide a quadrupole mass spectrometer that suffers no sensitivity deterioration when the scanning speed is increased.
Thus the quadrupole mass spectrometer according to the present invention includes:
an ion source;
a quadrupole filter including four rod electrodes for allowing ions having a preset mass number among ions generated by the ion source to pass through a space surrounded by the four rod electrodes;
an ion detector for detecting the ions passing through the space;
a quadrupole driver for applying a set of voltages to the four rod electrodes, where the set of voltages corresponds to the preset mass number;
a scanning controller for changing the set of voltages applied to the four rod electrodes to scan through a scanning range of the mass number; and
a field controller for producing an electrical field between the ion source and the quadrupole filter, where the magnitude of the electrical field depends on the scanning speed of the scanning controller.
The electrical field can control the kinetic energy of the ions from the ion source when they enter the quadrupole filter. In the present invention, the magnitude of the electrical field is controlled so that the kinetic energy of the ions entering the quadrupole filter becomes larger if the scanning speed is larger. When, for example, positive ions are generated in the ion source of a quadrupole mass spectrometer and the ion source box is grounded, a negative DC bias voltage is applied to the rod electrodes of the quadrupole filter. It should be noted here that the bias DC voltage is different from the DC voltage applied to the rod electrodes of the quadrupole filter in order to filter, or select, passing ions. Owing to the DC bias voltage, the ions entering the quadrupole filter are given a larger kinetic energy so that they pass through the quadrupole filter in a shorter time. The smaller the passing time is, the change in the voltage applied to the rod electrodes while the ions are passing is smaller. This decreases the possibility of proper ions that should pass through the quadrupole filter and should enter the ion detector dissipating in the course of the passage through the quadrupole filter due to the voltage change during the passage. Thus a larger number of ions can pass through the quadrupole filter in the quadrupole mass spectrometer of the present invention, so that the sensitivity of measurement is improved.